class_docstr: Class for fitting the Free Water Tensor Model 
docstr: Returns the free water diffusion volume fraction f 
docstr_tok: ["Returns","the","free","water","diffusion","volume","fraction","f"]
code: def f(self):
      """Returns the free water diffusion volume fraction f"""
      return self.model_params[..., 12]
  
imports: import warnings
  import numpy
  import scipy.optimize
  from dipy.reconst.base import ReconstModel
  from dipy.reconst.dti import TensorFit, design_matrix, decompose_tensor, _decompose_tensor_nan, from_lower_triangular, lower_triangular
  from dipy.reconst.dki import _positive_evals
  from dipy.reconst.vec_val_sum import vec_val_vect
  from dipy.core.ndindex import ndindex
  from dipy.core.gradients import check_multi_b
  from dipy.reconst.multi_voxel import multi_voxel_fit
label_desc: Use image node_id 2 for calling the FreeWaterTensorFit obj's underlying member method code with example usage: obj.f() and returns: self